Method for controlling handover, user equipment, base station, and radio communication system

ABSTRACT

Provided is a method for controlling a handover from a first base station to a second base station by a user equipment that performs radio communication over a communication channel formed by aggregating a plurality of component carriers, wherein scheduling information on each component carrier is transmitted to the user equipment according to either of a straight scheduling method and a cross scheduling method, and the method includes at the first base station, commanding first the user equipment to perform a handover from the first base station to the second base station on a second component carrier to be operated according to the straight scheduling method in the second base station, when it is determined that a handover on a first component carrier following the cross scheduling method is to be executed.

CROSS-REFERENCE TO RELATED APPLICATION

The application is a continuation of U.S. application Ser. No.13/579,398, filed Aug. 16, 2012, which is a national stage applicationof PCT/JP2011/051051 filed Jan. 21, 2011, and claims priority toJapanese Patent Application No. 2010-040464 filed Feb. 25, 2010, thecontents of each of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a method for controlling a handover, auser equipment, a base station, and a radio communication system.

BACKGROUND ART

In Long Term Evolution-Advanced (LTE-A), which is the next-generationcellular communication standard that is discussed in Third GenerationPartnership Project (3GPP), introduction of technology called carrieraggregation (CA) has been studied. The carrier aggregation is technologythat forms a communication channel between a user equipment (UE) and abase station (BS, or evolved Node B (eNB)) by aggregating a plurality offrequency bands that are supported in LTE, for example, and therebyimproves communication throughput. Each frequency band included in onecommunication channel by the carrier aggregation is called a componentcarrier (CC). The bandwidths of frequency bands that are available inLTE are 1.4 MHz, 3.0 MHz, 5.0 MHz, 10 MHz, 15 MHz, and 20 MHz.Accordingly, if five bands of 20 MHz are aggregated as componentcarriers, a communication channel of 100 MHz in total can be formed.

Component carriers that are included in one communication channel in thecarrier aggregation are not necessarily contiguous to one another in thefrequency direction. The mode in which component carriers are arrangedcontiguous to one another in the frequency direction is called acontiguous mode. On the other hand, the mode in which component carriersare arranged not contiguous to one another is called a non-contiguousmode.

Further, in the carrier aggregation, the number of component carriers inan uplink and the number of component carriers in a downlink are notnecessarily equal. The mode in which the number of component carriers inan uplink and the number of component carriers in a downlink are equalis called a symmetric mode. On the other hand, the mode in which thenumber of component carriers in an uplink and the number of componentcarriers in a downlink are not equal is called an asymmetric mode. Forexample, in the case of using two component carriers in an uplink andthree component carriers in a downlink, it is asymmetric carrieraggregation.

In radio communication of a conventional cellular system involving thecarrier aggregation, allocation information of communication resources(that is, scheduling information) is transmitted from a base stationfrom each user equipment over a downlink control channel (for example,PDCCH: Physical Downlink Control Channel). In radio communication notinvolving the carrier aggregation, the scheduling information can betransmitted according to two kinds of methods. A first method is amethod in which a conventional method is applied to individual componentcarriers as is. In other words, in the first method, a control channelfor transmitting scheduling information on data transmission in acertain component carrier is set inside the corresponding componentcarrier. In this disclosure, the first method is referred to as astraight scheduling method. On the other hand, in a second method, acontrol channel for transmitting scheduling information on datatransmission in a certain component carrier is set inside a componentcarrier different from the corresponding component carrier. According tothe second method, when communication resources used for transmission ofscheduling information are aggregated, a ratio of overhead occupied bycommunication resources is reduced. Thus, the second method can achievea higher throughput than the first method. In this disclosure, thesecond method is referred to as a “cross scheduling method.” Thestraight scheduling method and the cross scheduling method can becomplexly used within one communication channel. In other words, onechannel configuring a communication channel may be operated by the crossscheduling method, and another channel may be operated by the straightscheduling method.

A handover, which is a basic technique for achieving the mobility of auser equipment in the cellular communication standard, is one ofimportant subjects in LTE-A. In LTE, a user equipment measures acommunication quality over a channel with a serving base station (acurrently connected base station) and communication qualities withperipheral base stations and transmits a measurement report containingmeasurements to the serving base station. Receiving the measurementreport, the serving base station determines whether to execute ahandover based on the measurements contained in the report. Then, if itis determined that a handover is to be executed, a handover is carriedout among a source base station (the serving base station before ahandover), the user equipment, and a target base station (a serving basestation after a handover) in accordance with a prescribed procedure(e.g. cf. Patent Literature 1 below).

CITATION LIST Patent Literature

-   Patent Literature 1: JP 2009-232293A

SUMMARY OF INVENTION Technical Problem

However, no case has been reported where active consideration is givento how to carry out a handover procedure in a radio communicationinvolving the carrier aggregation.

In the existing handover procedure disclosed in Patent Literature 1,processing such as a handover request, approval of the request, issuanceof a handover command, and random access to a target base station isperformed under the assumption that one communication channel isconfigured with one component carrier. When the carrier aggregationtechnique is introduced, since component carriers are assumed to differin the channel quality from each other, it is desirable to execute ahandover for each component carrier. However, under the circumstances inwhich the cross scheduling method is employed, a data channel used toperform data transmission and a control channel used to transmitscheduling information related to data transmission can be located indifferent component carriers. For this reason, when a handover procedureof each component carrier is not appropriately controlled, data loss mayoccur due to loss or mismatching of scheduling information or the like.

In this regard, the present invention aims to provide a method forcontrolling a handover, a user equipment, a base station, and a radiocommunication system, which are novel and improved and are capable ofperforming a handover with no data loss even under the circumstances inwhich the cross scheduling method is employed.

Solution to Problem

According to an aspect of the present invention, there is provided amethod for controlling a handover from a first base station to a secondbase station by a user equipment that performs radio communication overa communication channel formed by aggregating a plurality of componentcarriers, wherein scheduling information on each component carrier istransmitted to the user equipment according to either of a straightscheduling method and a cross scheduling method, and the methodincludes, at the first base station, commanding first the user equipmentto perform a handover from the first base station to the second basestation on a second component carrier to be operated according to thestraight scheduling method in the second base station, when it isdetermined that a handover on a first component carrier following thecross scheduling method is to be executed.

Further, the first base station may command the user equipment toperform a handover on the second component carrier in which a channelfor transmitting scheduling information on the first component carrieris positioned before a handover on the first component carrier.

Further, the method may further include, at the first base station,commanding the user equipment to perform a handover on the firstcomponent carrier.

Further, the first component carrier and the second component carriermay be the same component carrier, and the method may further include,at the first base station, changing a scheduling informationtransmission method on the first component carrier from the crossscheduling method to the straight scheduling method before a firsthandover is executed.

Further, the first base station may change the scheduling informationtransmission method on the first component carrier from the crossscheduling method to the straight scheduling method according to achange request of the scheduling information transmission method from auser equipment that has executed measurement.

Further, the first base station may change the scheduling informationtransmission method on the first component carrier from the crossscheduling method to the straight scheduling method after a handoverrequest is confirmed by the second base station.

Further, the first component carrier and the second component carriermay be the same component carrier, and the second component carrier maybe operated according to the straight scheduling method in the secondbase station after a handover from the first base station to the secondbase station is performed.

Further, according to another aspect of the present invention, there isprovided a user equipment which includes a radio communication unit thatperforms radio communication with a base station over a communicationchannel formed by aggregating a plurality of component carriers, acontrol unit that controls a handover from a first base station to asecond base station by the radio communication unit, and a qualitymeasuring unit that measures a channel quality of the communicationchannel between the user equipment and the first base station, whereinscheduling information on each component carrier is transmittedaccording to either of a straight scheduling method and a crossscheduling method, and when it is determined that a handover on a firstcomponent carrier following the cross scheduling method is to beexecuted, a handover from the first base station to the second basestation on a second component carrier to be operated according to thestraight scheduling method in the second base station is first executedaccording to a command from the first base station.

Further, according to another aspect of the present invention, there isprovided a base station which includes a radio communication unit thatperforms radio communication with a user equipment over a communicationchannel formed by aggregating a plurality of component carriers, and acontrol unit that controls a handover to another base station by theuser equipment, wherein scheduling information on each component carrieris transmitted according to either of a straight scheduling method and across scheduling method, and when it is determined that a handover on afirst component carrier following the cross scheduling method is to beexecuted, the control unit commands first the user equipment to performa handover to the other base station on a second component carrier to beoperated according to the straight scheduling method in the other basestation.

Further, according to another aspect of the present invention, there isprovided a radio communication system which includes a user equipmentthat performs radio communication over a communication channel formed byaggregating a plurality of component carriers, a first base station thatprovides the user equipment with a service over the communicationchannel, and a second base station that is a target of a handover fromthe first base station by the user equipment, wherein schedulinginformation on each component carrier is transmitted according to eitherof a straight scheduling method and a cross scheduling method, and whenit is determined that a handover on a first component carrier followingthe cross scheduling method is to be executed, the first base stationcommands first the user equipment to perform a handover on a secondcomponent carrier to be operated according to the straight schedulingmethod in the second base station.

Advantageous Effects of Invention

As described above, a method for controlling a handover, a userequipment, a base station, and a radio communication system according tothe present invention can perform a handover with no data loss evenunder the circumstances in which cross scheduling can be performed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sequence chart to describe a flow of a typical handoverprocedure.

FIG. 2 is an explanatory view to describe an example of a structure ofcommunication resources.

FIG. 3 is an explanatory view to describe an example of an arrangementof a control channel included in communication resources.

FIG. 4 is an explanatory view to describe two kinds of schedulinginformation transmission methods.

FIG. 5 is a schematic view showing an outline of a radio communicationsystem according to an embodiment of the present invention.

FIG. 6 is a block diagram showing an exemplary configuration of a userequipment according to a first embodiment.

FIG. 7 is a block diagram showing an example of a more detailedconfiguration of a radio communication unit according to the firstembodiment.

FIG. 8 is a block diagram showing an exemplary configuration of a basestation according to the first embodiment.

FIG. 9 is a flowchart showing an example of the flow of a determinationprocess of a handover procedure by a base station according to the firstembodiment.

FIG. 10 is a sequence chart showing an example of the flow of a handoverprocedure according to the first embodiment.

FIG. 11A is a first explanatory view to further describe a scenariodescribed with reference to FIG. 10.

FIG. 11B is a second explanatory view to further describe a scenariodescribed with reference to FIG. 10.

FIG. 11C is a third explanatory view to further describe a scenariodescribed with reference to FIG. 10.

FIG. 11D is a fourth explanatory view to further describe a scenariodescribed with reference to FIG. 10.

FIG. 12 is a block diagram showing an exemplary configuration of a userequipment according to a second embodiment.

FIG. 13 is a block diagram showing an exemplary configuration of a basestation according to the second embodiment.

FIG. 14A is a sequence chart showing an example of the flow of a firstscenario of a handover procedure according to the second embodiment.

FIG. 14B is a sequence chart showing an example of the flow of a secondscenario of a handover procedure according to the second embodiment.

FIG. 15 is a block diagram showing an exemplary configuration of a userequipment according to a third embodiment.

FIG. 16 is a block diagram showing an exemplary configuration of a basestation according to the third embodiment.

FIG. 17 is a sequence chart showing an example of the flow of a handoverprocedure according to the third embodiment.

DESCRIPTION OF EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to the appended drawings. Note that,in this specification and the drawings, elements that have substantiallythe same function and structure are denoted with the same referencesigns, and repeated explanation is omitted.

Preferred embodiments of the present invention will be describedhereinafter in the following order.

1. Description of Related Art

1-1. Handover Procedure

1-2. Structure of Communication Resources

1-3. Scheduling Information Transmission Method

2. Outline of Radio Communication System

3. Description of First Embodiment

3-1. Exemplary Configuration of User Equipment

3-2. Exemplary Configuration of Base Station

3-3. Flow of Process

3-4. Summary of First Embodiment

4. Description of Second Embodiment

4-1. Exemplary Configuration of User Equipment

4-2. Exemplary Configuration of Base Station

4-3. Flow of Process

4-4. Summary of Second Embodiment

5. Description of Third Embodiment

5-1. Exemplary Configuration of User Equipment

5-2. Exemplary Configuration of Base Station

5-3. Flow of Process

5-4. Summary of Third Embodiment

1. Description of Related Art

(1-1. Handover Procedure)

A technique related to the present invention is described hereinafterwith reference to FIGS. 1 and 2. FIG. 1 shows a flow of a handoverprocedure in conformity with LTE in a radio communication not involvingthe carrier aggregation as an example of a typical handover procedure.In this example, a user equipment (UE), a source base station (sourceeNB), a target base station (target eNB), and a mobility managemententity (MIME) are involved in the handover procedure.

As a preliminary step toward a handover, the user equipment firstreports the channel quality of a communication channel between the userequipment and the source base station to the source base station (stepS2). The channel quality may be reported on a regular basis or when thechannel quality falls below a predetermined reference value. The userequipment can measure the channel quality of the communication channelwith the source base station by receiving a reference signal containedin a downlink channel from the source base station.

Then, the source base station determines the needs of measurement basedon the quality report received from the user equipment and, ifmeasurement is necessary, allocates measurement gaps to the userequipment (step S4).

Then, the user equipment searches for a downlink channel from aperipheral base station (i.e. performs cell search) during the periodsof the allocated measurement gaps (step S12). Note that the userequipment can recognize a peripheral base station to search according toa list that is provided in advance from the source base station.

When the user equipment acquires synchronization with a downlinkchannel, the user equipment performs measurement by using a referencesignal contained in the downlink channel (step S14). During this period,the source base station restricts an allocation of data communicationrelated to the user equipment so as to avoid occurrence of datatransmission by the user equipment.

Upon completion of the measurement, the user equipment transmits ameasurement report containing measurements to the source base station(step S22). The measurements contained in the measurement report may bethe average value or the central value of measured values over aplurality of times of measurement or the like. Further, the measurementsmay contain data about a plurality of frequency bands.

Receiving the measurement report, the source base station determineswhether or not to execute a handover based on the contents of themeasurement report. For example, when the channel quality of anotherbase station in the periphery is higher than the channel quality of thesource base station by a predetermined threshold or greater, it can bedetermined that a handover is necessary. In this case, the source basestation determines to carry out a handover procedure with the relevantanother base station as a target base station, and transmits a handoverrequest message to the target base station (step S24).

Receiving the handover request message, the target base stationdetermines whether it is possible to accept the user equipment accordingto the availability of a communication service offered by itself or thelike. When it is possible to accept the user equipment, the target basestation transmits a handover request confirm message to the source basestation (step S26).

Receiving the handover request confirm message, the source base stationtransmits a handover command to the user equipment (step S28). Then, theuser equipment acquires synchronization with the downlink channel of thetarget base station (step S32). After that, the user equipment makes arandom access to the target base station by using a random accesschannel in a given time slot (step S34). During this period, the sourcebase station forwards data addressed to the user equipment to the targetbase station (step S36). Then, after success in the random access, theuser equipment transmits a handover complete message to the target basestation (step S42).

Receiving the handover complete message, the target base stationrequests the MME to perform route update for the user equipment (stepS44). Upon updating the route of user data by the MME, the userequipment becomes able to communicate with another device through a newbase station (i.e. the target base station). Then, the target basestation transmits acknowledgement to the user equipment (step S46). Aseries of handover procedure thereby ends.

(1-2. Structure of Communication Resource)

FIG. 2 shows a structure of communication resources in LTE as an exampleof a structure of communication resources to which the present inventionis applicable. Referring to FIG. 2, the communication resources in LTEare segmented in the time direction into radio frames each having alength of 10 msec. One radio frame includes ten sub-frames, and onesub-frame is made up of two 0.5 ms slots. Further, one 0.5 ms slottypically includes seven OFDM symbols in the time direction. One unit ofcommunication resources including seven OFDM symbols in the timedirection and 12 sub carriers in the frequency direction is referred toas a resource block. In LTE, communication resources are allocated toeach user equipment in the time direction in units of sub frames or inunits of resource blocks. Further, one unit of communication resourcescorresponding to one OFDM symbol in the time direction and one subcarrier in the frequency direction is referred to as a resource element.In other words, one resource block corresponds 84 (=7×12) resourceelements. In conditions of the same bandwidth and the same time length,as the number of resource blocks allocated for data communicationincreases, the throughput of data communication increases.

Further, a synchronization sequence is included in a resource block at apredetermined position (typically, at the center of a band) in thefrequency direction. As the synchronization sequence, two kinds ofsynchronization sequences, that is, a primary synchronization sequence(PSS) and a secondary synchronization sequence (SSS), are used. A userequipment that has received the two kinds of synchronization sequencesin a cell search may discriminate between base stations and acquiresynchronization with a specific base station. The two kinds ofsynchronization sequences are arranged on sixth and seventh OFDM symbols#5 and #6 of one sub fame in the time direction. Further, an OFDM symbolfollowing the synchronization sequences may be used as a broadcastchannel for transmission of system information.

Further, a predetermined resource element in each resource block is usedfor transmission of a reference signal. A user equipment that hasreceived the reference signal may measure the communication quality inunits of resource blocks. Further, a scheduler of a base station decidesallocation of communication resources to a user equipment according tothe communication quality of each resource block that is measured by auser equipment in the downlink and measured by a base station in theuplink.

(1-3. Scheduling Information Transmission Method)

FIG. 3 shows an arrangement of a control channel as an example of anarrangement of a control channel included in communication resources.Unlike FIG. 2, in FIG. 3, a vertical axis represents a time direction,and a horizontal axis represents a frequency direction. Referring toFIG. 3, a communication resource of 12 sub carriers×1 sub frame isshown. 1 sub frame includes 14 OFDM symbols in the time direction. Inthese communication resources, a control channel used to transmitscheduling information, that is, a PDCCH, is arranged in a maximum of 3OFDM symbols at the head of a sub frame. In addition to the schedulinginformation, information used to designate a modulation scheme, powercontrol information, and the like may be transmitted over the PDCCH. Auser equipment recognizes communication resources used for each deviceto transmit or receive data with reference to the scheduling informationon the control channel. Transmission and reception of data is performedover a physical downlink shared channel (PDSCH) which is a data channelarranged in remaining OFDM symbols of the sub frame.

Here, in radio communication involving the carrier aggregation, aplurality of component carriers configure one communication channel.Typically, each component carrier includes a control channel. However,in order to improve the throughput by allocating more resource blocksfor data communication (that is, by reducing the overhead), a techniquecalled cross scheduling (or cross carrier scheduling) which will bedescribed later with reference to FIG. 4 may be used.

FIG. 4 is an explanatory view to describe two kinds of schedulinginformation transmission methods in radio communication involving thecarrier aggregation. Referring to FIG. 4, 3 component carriers CC1 toCC3 configure one communication channel. Of these, each of the componentcarriers CC1 and CC2 includes a control channel (PDCCH). Schedulinginformation for data communication in the component carrier CC1 istransmitted over the control channel of the component carrier CC1.Scheduling information for data communication in the component carrierCC2 is transmitted over the control channel of the component carrierCC2. Meanwhile, the component carrier CC3 does not include a controlchannel. Scheduling information for data communication in the componentcarrier CC3 is transmitted over the control channel of the componentcarrier CC2. Thus, in the example of FIG. 4, the component carriers CC1and CC2 follow the straight scheduling method, and the component carrierCC3 follows the cross scheduling method. The component carrier followingthe cross scheduling method is also referred to as an extension carrier.Further, in this disclosure, a component carrier including a controlchannel for an extension carrier is referred to as a master of theextension carrier. In the example of FIG. 4, the component carrier CC2is the master of the component carrier CC3.

2. Outline of Radio Communication System

FIG. 5 is a schematic view showing an outline of a radio communicationsystem 1 according to an embodiment of the present invention. Referringto FIG. 5, the radio communication system 1 includes a user equipment100, a base station 200 a and a base station 200 b. It is assumed thatthe base station 200 a is a serving base station for the user equipment100.

The user equipment 100 is located inside a cell 202 a where a radiocommunication service is provided by the base station 200 a. The userequipment 100 can perform a data communication with another userequipment (not shown) via the base station 200 a over a communicationchannel formed by aggregating a plurality of component carriers (i.e. bycarrier aggregation). However, because the distance between the userequipment 100 and the base station 200 a is not short, there is apossibility that a handover is required for the user equipment 100.Further, the user equipment 100 is located inside a cell 202 b where aradio communication service is provided by the base station 200 b.Therefore, the base station 200 b can be a candidate for a target basestation for a handover of the user equipment 100.

The base station 200 a can communicate with the base station 200 bthrough a backhaul link (e.g. X2 interface). Various kinds of messagesin the handover procedure as described with reference to FIG. 1,scheduling information related to the user equipment belonging to eachcell or the like, for example, can be transmitted and received betweenthe base station 200 a and the base station 200 b. Further, the basestation 200 a and the base station 200 b can communicate with the MME,which is an upper node, through S1 interface, for example.

Here, it is assumed that a need to perform a handover to the basestation 200 b arises when the user equipment 100 performs radiocommunication involving the carrier aggregation with the base station200 a. In this case, for example, an attempt to perform a handover ismade first on a component carrier having the worst quality between theuser equipment 100 and the base station 200 a. At this time, however,when the corresponding component carrier is an extension carrier, ahandover is performed before a component carrier of a master, and thusdata loss may occur due to loss or mismatching of schedulinginformation. For this reason, it is desirable to control a handoverprocedure not to cause data loss under the circumstances in which thecross scheduling method is used as in first to third embodiments of thepresent invention which will be described in detail in the next section.

It should be noted that, when there is no particular need to distinguishbetween the base station 200 a and the base station 200 b in thefollowing description of the specification, they are collectivelyreferred to as a base station 200 by omitting the alphabetical letter atthe end of the reference symbol. The same applies to the other elements.

3. Description of First Embodiment

Next, a first embodiment of the present invention will be described withreference to FIGS. 6 to 11D.

(3-1. Exemplary Configuration of User Equipment)

FIG. 6 is a block diagram showing an exemplary configuration of the userequipment 100 according to the present embodiment. Referring to FIG. 6,the user equipment 100 includes a radio communication unit 110, a signalprocessing unit 150, a control unit 160, and a measurement unit 170.

(Radio Communication Unit)

The radio communication unit 110 performs a radio communication with thebase station 200 over a communication channel formed by aggregating aplurality of component carriers with use of the carrier aggregationtechnology.

FIG. 7 is a block diagram showing an example of a more detailedconfiguration of the radio communication unit 110. Referring to FIG. 7,the radio communication unit 110 includes an antenna 112, a switch 114,a low noise amplifier (LNA) 120, a plurality of down-converters 122 a to122 c, a plurality of filters 124 a to 124 c, a plurality ofanalogue-to-digital converters (ADCs) 126 a to 126 c, a demodulationunit 128, a modulation unit 130, a plurality of digital-to-analogueconverters (DACs) 132 a to 132 c, a plurality of filters 134 a to 134 c,a plurality of up-converters 136 a to 136 c, a combiner 138, and a poweramplifier (PA) 140.

The antenna 112 receives a radio signal transmitted from the basestation 200 and outputs the received signal to the LNA 120 through theswitch 114. The LNA 120 amplifies the received signal. Thedown-converter 122 a and the filter 124 a separate a baseband signal ofthe first component carrier (CC1) from the received signal amplified bythe LNA 120. Then, the separated baseband signal is converted to adigital signal by the ADC 126 a and output to the demodulation unit 128.Likewise, the down-converter 122 b and the filter 124 b separate abaseband signal of the second component carrier (CC2) from the receivedsignal amplified by the LNA 120. Then, the separated baseband signal isconverted to a digital signal by the ADC 126 b and output to thedemodulation unit 128. Further, the down-converter 122 c and the filter124 c separate a baseband signal of the third component carrier (CC3)from the received signal amplified by the LNA 120. Then, the separatedbaseband signal is converted to a digital signal by the ADC 126 c andoutput to the demodulation unit 128. After that, the demodulation unit128 generates a data signal by demodulating the baseband signals of therespective component carriers and outputs the data signal to the signalprocessing unit 150.

Further, when a data signal is input from the signal processing unit150, the modulation unit 130 modulates the data signal and generatesbaseband signals of the respective component carriers. Among thosebaseband signals, the baseband signal of the first component carrier(CC1) is converted to an analog signal by the DAC 132 a. Then, afrequency component corresponding to the first component carrier in atransmission signal is generated from the analog signal by the filter134 a and the up-converter 136 a. Likewise, the baseband signal of thesecond component carrier (CC2) is converted to an analog signal by theDAC 132 b. Then, a frequency component corresponding to the secondcomponent carrier in the transmission signal is generated from theanalog signal by the filter 134 b and the up-converter 136 b. Further,the baseband signal of the third component carrier (CC3) is converted toan analog signal by the DAC 132 c. Then, a frequency componentcorresponding to the third component carrier in the transmission signalis generated from the analog signal by the filter 134 c and theup-converter 136 c. After that, the generated frequency componentscorresponding to the three component carriers are combined by thecombiner 138, and the transmission signal is formed. The PA 140amplifies the transmission signal and outputs the transmission signal tothe antenna 112 through the switch 114. Then, the antenna 112 transmitsthe transmission signal as a radio signal to the base station 200.

Although the case where the radio communication unit 110 handles threecomponent carriers is described in FIG. 7, the number of componentcarriers handled by the radio communication unit 110 may be two, or fouror more.

Further, instead of processing the signals of the respective componentcarriers in the analog region as in the example of FIG. 7, the radiocommunication unit 110 may process the signals of the respectivecomponent carriers in the digital region. In the latter case, at thetime of reception, a digital signal converted by one ADC is separatedinto the signals of the respective component carriers by a digitalfilter. Further, at the time of transmission, after digital signals ofthe respective component carriers are frequency-converted and combined,the signal is converted into an analog signal by one DAC. The load ofthe ADC and the DAC is generally smaller when processing the signals ofthe respective component carriers in the analog region. On the otherhand, when processing the signals of the respective component carriersin the digital region, a sampling frequency for AD/DA conversion ishigher, and the load of the ADC and the DAC can thereby increase.

(Signal Processing Unit)

Referring back to FIG. 6, an example of a configuration of the userequipment 100 is further described.

The signal processing unit 150 performs signal processing such asdeinterleaving, decoding or error correction on the demodulated datasignal that is input from the radio communication unit 110. Then, thesignal processing unit 150 outputs the processed data signal to an upperlayer. Further, the signal processing unit 150 performs signalprocessing such as encoding or interleaving on the data signal that isinput from the upper layer. Then, the signal processing unit 150 outputsthe processed data signals to the radio communication unit 110.

(Control Unit)

The control unit 160 controls the overall functions of the userequipment 100 using a processing device such as a central processingunit (CPU) or a digital signal processor (DSP). For example, the controlunit 160 controls a timing of data communication by the radiocommunication unit 110 for each component carrier according toscheduling information which is received from the base station 200 bythe radio communication unit 110. More specifically, for example, thecontrol unit 160 refers to scheduling information on a control channelof a component carrier following the straight scheduling method amongcomponent carriers configuring a communication channel between the upperequipment 100 and the base station 200. This scheduling information mayinclude information on an extension carrier following the crossscheduling method in addition to information on a communication resourceof the same component carrier. Thus, when information on an extensioncarrier is included in scheduling information, the control unit 160controls a timing of data communication on a data channel of anextension carrier according to the corresponding information. Further,the control unit 160 controls a timing of data communication on a datachannel of a component carrier following the straight scheduling methodaccording to information on communication resources of the samecomponent carrier as the control channel. In addition, the control unit160 causes the user equipment 100 to operate in the same way as the userequipment in the handover procedure described with reference to FIG. 1.

(Measurement Unit)

The measurement unit 170 measures the channel quality for each of thecomponent carriers by using a reference signal from the base station 200according to control from the control unit 160, for example. Further,the measurement unit 170 executes measurement for a handover withrespect to each of the component carriers by using the measurement gapswhich are allocated by the base station 200. A result of the measurementexecuted by the measurement unit 170 is converted to a predeterminedformat for a measurement report by the control unit 160 and transmittedto the base station 200 through the radio communication unit 110. Afterthat, the base station 200 determines, based on the measurement report,whether a handover should be executed or not for the user equipment 100.

(3-2. Exemplary Configuration of Base Station)

FIG. 8 is a block diagram showing an exemplary configuration of the basestation 200 according to the embodiment. Referring to FIG. 8, the basestation 200 includes a radio communication unit 210, an interface unit250, a component carrier (CC) management unit 260, and a control unit280.

(Radio Communication Unit)

A specific configuration of the radio communication unit 210 may besimilar to the configuration of the radio communication unit 110 of theuser equipment 100 which is described above with reference to FIG. 7,although the number of component carriers to be supported, therequirements of processing performance or the like are different. Theradio communication unit 210 performs a radio communication with theuser equipment over a communication channel which is formed byaggregating a plurality of component carriers with use of the carrieraggregation technology.

(Interface Unit)

The interface unit 250 mediates a communication between the radiocommunication unit 210 or the control unit 280 and an upper node throughthe S1 interface illustrated in FIG. 5, for example. Further, theinterface unit 250 mediates a communication between the radiocommunication unit 210 or the control unit 280 and another base stationthrough the X2 interface illustrated in FIG. 5, for example.

(CC Management Unit)

The CC management unit 260 holds data that indicates which componentcarrier each user equipment is using for communication with respect toeach of the user equipments belonging to the cell of the base station200. Such data can be updated by the control unit 280 when an additionaluser equipment joins the cell of the base station 200 or when aconnected user equipment changes its component carriers. Thus, thecontrol unit 280 can recognize which component carrier the userequipment 100 is using by referring to the data held by the CCmanagement unit 260.

(Control Unit)

The control unit 280 controls the overall functions of the base station200 using the processing device such as a CPU or a DSP. For example, thecontrol unit 280 allocates communication resources for datacommunication to the user equipment 100 and another user equipment, andthen transmits scheduling information over a control channel of acomponent carrier corresponding to a scheduling information transmissionmethod.

More specifically, the control unit 280 transmits scheduling informationon communication resources in a component carrier following the straightscheduling method over a control channel of the same component carrier.Further, the control unit 280 transmits scheduling information oncommunication resources in an extension carrier over a control channelof a different component carrier which is a master.

Further, the control unit 280 controls a handover to another basestation by the user equipment 100. More specifically, for example, whenit is determined that a handover on an extension carrier needs to beperformed, the control unit 280 first commands the user equipment 100 toperform a handover to a target base station on a component carrier to beoperated according to straight scheduling method in the target basestation. For example, it is assumed that the base station 200 and thetarget base station share a scheduling information transmission methodfor each component carrier (for each operating frequency band). In thiscase, for example, when it is determined that a handover on an extensioncarrier needs to be performed, the control unit 280 commands the userequipment 100 to execute a handover on a component carrier which is amaster of the corresponding extension carrier before a handover on theextension carrier. Then, after completing a handover on the componentcarrier of the master, the control unit 280 commands the user equipment100 to execute a handover on the extension carrier. Further, whenanother available component carrier is present, the control unit 280 maytemporarily transmit scheduling information on the extension carrierover a control channel of another component carrier until the handoveron the extension carrier is completed after the handover on thecomponent carrier of the master starts. In addition, the control unit280 causes the base station 200 to operate in the same way as the sourcebase station or the target base station in the handover proceduredescribed above with reference to FIG. 1.

(3-3. Flow of Process)

FIG. 9 is a flowchart showing an example of the flow of a determinationprocess of a handover procedure by the control unit 280 of the basestation 200 according to the present embodiment.

Referring to FIG. 9, the control unit 280 receives a measurement reportfrom the user equipment 100 through the radio communication unit 210(step S102). Next, the control unit 280 determines whether or not thereis a component carrier whose quality does not satisfy a predeterminedcriterion based on content of the measurement report (step S104). Here,when there is no component carrier whose quality does not satisfy apredetermined criterion, the process ends. However, when there is acomponent carrier whose quality does not satisfy a predeterminedcriterion, the process proceeds to step S106. Next, the control unit 280determines whether or not the component carrier whose quality does notsatisfy a predetermined criterion is a component carrier to be operatedaccording to the cross scheduling method (step S106). Here, when thecorresponding component carrier is a component carrier to be operatedaccording to the cross scheduling method, the process proceeds to stepS108. However, when the corresponding component carrier is not acomponent carrier to be operated according to the cross schedulingmethod, the process proceeds to step S110. In step S108, the controlunit 280 decides to perform a handover on a component carrier which is amaster of the corresponding component carrier before the componentcarrier to be operated according to the cross scheduling method (stepS108). Meanwhile, in step S110, the control unit 280 decides a handoveron a component carrier determined as having a quality that does notsatisfy a predetermine criterion (step S110).

FIG. 10 is a sequence chart showing an example of the flow of a handoverprocedure according to the present embodiment. In a scenario of FIG. 10,it is assumed that a handover procedure is performed among the userequipment 100, the base station 200 a serving as the source basestation, and the base station 200 b serving as the target base station.Further, a procedure (steps S2 to S14) up to the measurement in the userequipment in the general handover procedure illustrated in FIG. 1 is notparticularly different, and thus a description thereof will not be made.

Referring to FIG. 10, the user equipment 100 first transmits ameasurement report on a plurality of component carriers configuring acommunication channel to the base station 200 a (step S120). Next, thebase station 200 a determines whether or not a handover is necessary anda handover procedure for each component carrier based on the receivedmeasurement report as described above with reference to FIG. 9 (stepS130).

In the scenario of FIG. 10, for example, it is assumed that a handoveron a component carrier to be operated as an extension carrier isnecessary. In this case, the base station 200 a transmits a handoverrequest message for requesting a handover on a component carrier whichis a master of the corresponding component carrier to the base station200 b (step S144). The base station 200 b that has received the handoverrequest message determines whether or not the user equipment 100 can beaccepted, for example, based on availability of a communication serviceoffered by itself. Then, when it is determined that the user equipment100 can be accepted, the base station 200 b transmits a handover requestconfirm message to the base station 200 a (step S146). The base station200 a that has received the handover request confirm message transmits ahandover command on a component carrier of a master to the userequipment 100 (step S148). Thus, a handover on a component carrier of amaster is executed among the user equipment 100 that has received thehandover command, the base station 200 a, the base station 200 b, andthe MME (step S150). In step S150, for example, synchronization with thetarget base station, random access to the target base station,transmission of a handover complete message, a route update,transmission of acknowledgement, and the like are performed, similarlyto the process described above with reference to FIG. 1.

Next, the base station 200 a transmits a handover request message forrequesting a handover on a component carrier to be operated as anextension carrier to the base station 200 b (step S164). The basestation 200 b that has received the handover request message determineswhether or not the user equipment 100 can be accepted, for example,based on availability of a communication service offered by itself.Then, when it is determined that the user equipment 100 can be accepted,the base station 200 b transmits a handover request confirm message tothe base station 200 a (step S166). The base station 200 a that hasreceived the handover request confirm message transmits a handovercommand on a component carrier to be operated as an extension carrier tothe user equipment 100 (step S168). Thus, a handover on thecorresponding component carrier is executed among the user equipment 100that has received the handover command, the base station 200 a, the basestation 200 b, and the MME (step S170).

Thereafter, when a handover on another component carrier is necessary, ahandover is performed on each component carrier in the same way as theabove-described procedure.

FIGS. 11A to 11D are explanatory views to further describe the scenariodescribed above with reference to FIG. 10.

Referring to FIG. 11A, three component carriers CC1 to CC3 configuring acommunication channel between the user equipment 100 and the basestation 200 a are depicted. At a point in time of FIG. 11A, the basestation 200 a functions as the serving base station of the userequipment 100. Further, among the three component carriers, thecomponent carriers CC1 and CC2 are operated according to the straightscheduling method. The component carrier CC3 is operated according tothe cross scheduling method. In this situation, when it is determinedthat a handover to the base station 200 b is necessary on the componentcarrier CC3, a handover is first performed on the component carrier CC2which is a master of the component carrier CC3. This is the same evenwhen it is determined that a handover is necessary on the componentcarrier CC2.

FIG. 11B shows a state after the user equipment 100 has performed ahandover from the base station 200 a to the base station 200 b on thecomponent carrier CC2 (after step S150 of FIG. 10 is completed). In FIG.11B, among the three component carriers of the user equipment 100, thecomponent carriers CC1 and CC3 remain connected to the base station 200a, and the component carrier CC2 remains connected to the base station200 b. Further, the component carrier CC2 is operated by the straightscheduling method even between the user equipment 100 and the basestation 200 b. Meanwhile, a master of the component carrier CC3 istemporarily changed to the component carrier CC1. For example, thechange of the master of the extension carrier may be performed such thata change notice is transmitted from the base station 200 a to the userequipment 100 (and another user equipment), and then the base station200 a changes a channel in which scheduling information on an extensioncarrier is to be included.

Further, FIG. 11C shows a state after the user equipment 100 hasperformed a handover from the base station 200 a to the base station 200b on the component carrier CC3 (after step S170 of FIG. 10 iscompleted). In FIG. 11C, among the three component carriers of the userequipment 100, the component carrier CC1 remains connected to the basestation 200 a, and the component carriers CC2 and CC3 remain connectedto the base station 200 b. Further, the component carrier CC2 functionsas the master of the component carrier CC3 in the communication channelbetween the user equipment 100 and the base station 200 b.

Further, FIG. 11D shows a state after the user equipment 100 hasperformed a handover from the base station 200 a to the base station 200b on the component carrier CC1. In FIG. 11D, all of the three componentcarriers of the user equipment 100 remain connected to the base station200 b.

(3-4. Summary of First Embodiment)

The first embodiment of the present invention has been described so farwith reference to FIGS. 6 to 11D. According to the present embodiment,in radio communication involving the carrier aggregation, when it isdetermined that a handover needs to be executed on an extension carrierfollowing the cross scheduling method, a handover is first executed on acomponent carrier to be operated according to the straight schedulingmethod in the target base station. In this case, for example, acomponent carrier on which a handover is first executed is a componentcarrier functioning as a master of an extension carrier. Thereafter, ahandover is performed on a component carrier to be operated according tothe cross scheduling method in the target base station. In thissequence, a component carrier functioning as a master of an extensioncarrier and the extension carrier are consecutively handed over, andthus a risk that data loss will occur due to loss or mismatching ofscheduling information is reduced or eliminated. Accordingly, a seamlesshandover can be implemented even under the circumstances in which thecross scheduling method is performed. Furthermore, in the presentembodiment, a scheduling information transmission method of eachcomponent carrier need not be changed for a handover procedure, and thusthe impact on a system is small.

4. Description of Second Embodiment

Next, a second embodiment of the present invention will be describedwith reference to FIGS. 12 to 14B.

(4-1. Exemplary Configuration of User Equipment)

FIG. 12 is a block diagram showing an exemplary configuration of a userequipment 300 according to the present embodiment. Referring to FIG. 12,the user equipment 300 includes a radio communication unit 110, a signalprocessing unit 150, a control unit 360, and a measurement unit 170.

(Control Unit)

The control unit 360 controls the overall functions of the userequipment 300 using the processing device such as a CPU or a DSP. Forexample, the control unit 360 controls a timing of data communication bythe radio communication unit 110 for each component carrier according toscheduling information which is received from a base station 400 by theradio communication unit 110, similarly to the control unit 160according to the first embodiment. Further, in the present embodiment,for example, when it is determined that the quality of an extensioncarrier operated by the cross scheduling method is being lowered basedon the result of measurement made by the measurement unit 170, thecontrol unit 360 transmits a change request of the schedulinginformation transmission method to the base station 400. This is done toprepare for a handover on a component carrier whose quality isdecreasing by changing the scheduling information transmission method ofthe corresponding component carrier from the cross scheduling method tothe straight scheduling method. Further, even when the control unit 360does not request a change of the scheduling information transmissionmethod, when a change of the scheduling information transmission methodis notified of by the base station 400, the control unit 360 changes acomponent carrier operating method according to the correspondingnotice. In addition, the control unit 360 causes the user equipment 300to operate in the same way as the user equipment in the handoverprocedure described above with reference to FIG. 1.

(4-2. Exemplary Configuration of Base Station)

FIG. 13 is a block diagram showing an exemplary configuration of thebase station 400 according to the present embodiment. Referring to FIG.13, the base station 400 includes a radio communication unit 210, aninterface unit 250, a CC management unit 260, and a control unit 480.

(Control Unit)

The control unit 480 controls the overall functions of the base station400 using the processing device such as a CPU or a DSP. For example, thecontrol unit 480 allocates a communication resource for datacommunication to the user equipment 300 and other user equipment, andthen transmits scheduling information over a control channel of acomponent carrier corresponding to a scheduling information transmissionmethod, similarly to the control unit 280 according to the firstembodiment.

Further, the control unit 480 controls a handover to another basestation by the user equipment 300. For example, in the presentembodiment, when it is determined that a handover on a component carrierwhich is an extension carrier needs to be executed, after the targetbase station confirms a request of a handover on the correspondingcomponent carrier, the control unit 480 changes the schedulinginformation transmission method on the extension carrier from the crossscheduling method to the straight scheduling method. Further, thecontrol unit 480 notifies the user equipment 300 of the change of thescheduling information transmission method. Further, when the changerequest of the scheduling information transmission method is receivedfrom the user equipment 300, the control unit 480 changes the schedulinginformation transmission method according to the corresponding request.Further, when the request from the user equipment 300 competes withanother user equipment (for example, when it leads to a reduction in thethroughput of communication of another user equipment with a highpriority), the control unit 480 may deny the request from the userequipment 300. Thereafter, the control unit 480 executes a handover on acomponent carrier on which it is determined that a handover needs to beexecuted. In addition, the control unit 480 causes the base station 400to operate in the same way as the source base station or the target basestation in the handover procedure described above with reference to FIG.1.

(4-3. Flow of Process)

Next, two scenarios of handover procedures according to the presentembodiment will be described. In these scenarios, it is assumed that thehandover procedure is performed among the user equipment 300, a basestation 400 a serving as the source base station, and a base station 400b serving as the target base station. Further, a procedure (steps S2 toS14) up to the measurement in the user equipment in the general handoverprocedure illustrated in FIG. 1 is not particularly different, and thusa description thereof will not be made.

FIG. 14A is a sequence chart showing an example of the flow of a firstscenario of a handover procedure according to the present embodiment.

Referring to FIG. 14A, the user equipment 300 that has completed themeasurement first evaluates the quality of a communication channelbetween the user equipment 300 and the base station 400 a for eachcomponent carrier (step S210). Then, when it is determined that thequality of an extension carrier operated by the cross scheduling methodis being lowered, the user equipment 300 transmits a change request tochange the scheduling information transmission method from the crossscheduling method to the straight scheduling method to the base station400 a (step S212). Next, the base station 400 a changes the schedulinginformation transmission method on the extension carrier according tothe request, and transmits an acknowledgement (ACK) to the userequipment 300 (step S214). Next, the user equipment 300 transmits ameasurement report to the base station 400 a (step S222). Next, the basestation 400 a transmits a handover request message for requesting ahandover on a component carrier on which it is determined that ahandover is necessary based on the measurement report to the basestation 400 b (step S224). Here, for example, the component carrier onwhich it is determined that a handover is necessary is the componentcarrier whose scheduling information transmission method has beenchanged from the cross scheduling method to the straight schedulingmethod in steps S212 to S214. The base station 400 b that has receivedthe handover request message determines whether or not the userequipment 300 can be accepted, for example, based on availability of acommunication service offered by itself. Then, when it is determinedthat the user equipment 300 can be accepted, the base station 400 btransmits a handover request confirm message to the base station 400 a(step S226). The base station 400 a that has received the handoverrequest confirm message transmits a handover command to the userequipment 300 (step S228). Thus, a handover is executed among the userequipment 300 that has received the handover command, the base station400 a, the base station 400 b, and the MIME (step S230). In step S230,for example, synchronization with the target base station, random accessto the target base station, transmission of a handover complete message,a route update, transmission of acknowledgement, and the like areperformed, similarly to the process described above with reference toFIG. 1.

FIG. 14B is a sequence chart showing an example of the flow of a secondscenario of a handover procedure according to the present embodiment.

Referring to FIG. 14B, the user equipment 300 that has completed themeasurement first transmits a measurement report to the base station 400a (step S310). Next, the base station 400 a evaluates the quality of acommunication channel between the user equipment 300 and the basestation 400 a for each component carrier (step S312). Next, the basestation 400 a transmits a handover request message for requesting ahandover on a component carrier on which it is determined that ahandover is necessary to the base station 400 b (step S324). The basestation 400 b that has received the handover request message determineswhether or not the user equipment 300 can be accepted, for example,based on availability of a communication service offered by itself.Then, when it is determined that the user equipment 300 can be accepted,the base station 400 b transmits a handover request confirm message tothe base station 400 a (step S326). Next, when the component carrier onwhich a handover request has been confirmed is an extension carrieroperated by the cross scheduling method, the base station 400 atransmits a notice representing that the scheduling informationtransmission method on the corresponding extension carrier is to bechanged from the cross scheduling method to the straight schedulingmethod to the user equipment 300 (step S330). Next, when acknowledgementis received from the user equipment 300 (step S332), the base station400 a changes the scheduling information transmission method on theextension carrier to the straight scheduling method. Then, the basestation 400 a transmits a handover command to the user equipment 300(step S334). Thus, a handover is executed among the user equipment 300that has received the handover command, the base station 400 a, the basestation 400 b, and the MME (step S340).

(4-4. Summary of Second Embodiment)

The second embodiment of the present invention has been described so farwith reference to FIGS. 12 to 14B. According to the present embodiment,in radio communication involving the carrier aggregation, when it isdetermined that a handover needs to be executed on an extension carrierfollowing the cross scheduling method, the scheduling informationtransmission method on the corresponding extension carrier is changed tothe straight scheduling method before a handover is executed. As aresult, a component carrier on which a handover is first executed isoperated according to the straight scheduling method. Accordingly, arisk that data loss will occur due to loss or mismatching of schedulinginformation is reduced or eliminated. Furthermore, in the presentembodiment, since an extension carrier and a component carrierfunctioning as a master of the extension carrier need not beconsecutively handed over, for example, when the quality of thecomponent carrier of the master is good, a connection of thecorresponding component carrier having a good quality with the sourcebase station can be maintained.

5. Description of Third Embodiment

Next, a third embodiment of the present invention will be described withreference to FIGS. 15 to 17.

(5-1. Exemplary Configuration of User Equipment)

FIG. 15 is a block diagram showing an exemplary configuration of a userequipment 500 according to the present embodiment. Referring to FIG. 15,the user equipment 500 includes a radio communication unit 110, a signalprocessing unit 150, a control unit 560, and a measurement unit 170.

(Control Unit)

The control unit 560 controls the overall functions of the userequipment 500 using the processing device such as a CPU or a DSP. Forexample, the control unit 560 controls a timing of data communication bythe radio communication unit 110 for each component carrier according toscheduling information which is received from a base station 600 by theradio communication unit 110, similarly to the control unit 160according to the first embodiment. Further, in the present embodiment,for example, when a handover command is received from the base station600 on a component carrier operated according to the cross schedulingmethod, the control unit 560 performs access using a component carrierfollowing the straight scheduling method when performing access to thetarget base station through the corresponding component carrier. Inother words, the control unit 560 changes an operating method on anextension carrier from the cross scheduling method to the straightscheduling method at the time of access to the target base station. Inaddition, the control unit 560 causes the user equipment 500 to operatein the same way as the user equipment in the handover proceduredescribed above with reference to FIG. 1.

(5-2. Exemplary Configuration of Base Station)

FIG. 16 is a block diagram showing an exemplary configuration of thebase station 600 according to the present embodiment. Referring to FIG.16, the base station 600 includes a radio communication unit 210, aninterface unit 250, a CC management unit 260, and a control unit 680.

(Control Unit)

The control unit 680 controls the overall functions of the base station600 using the processing device such as a CPU or a DSP. For example, thecontrol unit 680 allocates a communication resource for datacommunication to the user equipment 500 and another user equipment, andthen transmits scheduling information over a control channel of acomponent carrier corresponding to a scheduling information transmissionmethod, similarly to the control unit 280 according to the firstembodiment.

Further, the control unit 680 controls a handover to another basestation by the user equipment 500. For example, in the presentembodiment, when it is determined that a handover on a component carrierwhich is an extension carrier needs to be executed, the control unit 680detects a component carrier operated according to the straightscheduling method in the target base station. For example, the componentcarrier operated according to the straight scheduling method in thetarget base station can be detected by receiving system informationwhich is transmitted through an X2 interface illustrated in FIG. 5 ortransmitted over a broadcast channel from the target base station. Then,the control unit 680 transmits a handover command for causing anextension carrier on which it is determined that a handover needs to beexecuted to be handed over to a component carrier operated according tothe straight scheduling method in the target base station to the userequipment 500. In addition, the control unit 680 causes the base station600 to operate in the same way as the source base station or the targetbase station in the handover procedure described above with reference toFIG. 1.

(5-3. Flow of Process)

FIG. 17 is a sequence chart showing an example of the flow of a handoverprocedure according to the present embodiment. In the scenario of FIG.17, it is assumed that the handover procedure is performed among theuser equipment 500, a base station 600 a serving as the source basestation, and a base station 600 b serving as the target base station.Further, a procedure (steps S2 to S14) up to the measurement in the userequipment in the general handover procedure illustrated in FIG. 1 is notparticularly different, and thus a description thereof will not be made.

Referring to FIG. 17, the user equipment 500 first transmits ameasurement report on a plurality of component carriers configuring acommunication channel to the base station 600 a (step S420). Next, whenit is determined that a handover needs to be executed on a componentcarrier which is an extension carrier based on the measurement report,the base station 600 a detects a component carrier operated according tothe straight scheduling method in the base station 600 b (step S430).Next, the base station 600 a transmits a handover request message forrequesting a handover to the detected component carrier to the basestation 600 b (step S444). The base station 600 b that has received thehandover request message determines whether or not the user equipment500 can be accepted, for example, based on availability of acommunication service offered by itself. Then, when it is determinedthat the user equipment 500 can be accepted, the base station 600 btransmits a handover request confirm message to the base station 600 a(step S446). The base station 600 a that has received the handoverrequest confirm message transmits a handover command for causing anextension carrier on which it is determined that a handover needs to beexecuted to be handed over to a component carrier used in the basestation 600 b detected in step S430 to the user equipment 500 (stepS448). Thus, a handover is executed among the user equipment 500 thathas received the handover command, the base station 600 a, the basestation 600 b, and the MME while changing the scheduling method from thecross scheduling method to the straight scheduling method (step S450).More specifically, for example, the user equipment 500 changes anoperating frequency band of the extension carrier to a frequency band ofthe component carrier designated in the handover command, and makes anattempt to acquire synchronization with the base station 600 b and makerandom access to the base station 600 b. Then, when random access issuccessfully made, a handover complete message is transmitted from theuser equipment 500 to the base station 600 b, and a route is updated bythe MME. Thereafter, the user equipment 500 can perform communicationwith another device over a new component carrier between itself and thebase station 600 b. At this time, since the new component carrierfollows the straight scheduling method, the user equipment 500 performscommunication via the base station 600 b according to schedulinginformation on a control channel of the same component carrier.

(5-4. Summary of Third Embodiment)

The third embodiment of the present invention has been described so farwith reference to FIGS. 15 to 17. According to the present embodiment,in radio communication involving the carrier aggregation, when it isdetermined that a handover needs to be executed on a component carrierfollowing the cross scheduling method, a handover is executed on thecorresponding component carrier. Further, in the target base station,after a handover is completed, the corresponding component carrier isoperated according to the straight scheduling method. Accordingly, arisk that data loss will occur due to loss or mismatching of schedulinginformation is reduced or eliminated. Furthermore, in the presentembodiment, since an extension carrier and a component carrierfunctioning as a master of the extension carrier need not beconsecutively handed over, for example, when the quality of thecomponent carrier of the master is good, a connection of thecorresponding component carrier having a good quality with the sourcebase station can be maintained.

As described above, according to the three embodiments described in thisdisclosure, by appropriately controlling a handover procedure of eachcomponent carrier, a handover can be performed with no data loss evenunder the circumstances cross scheduling can be performed.

The preferred embodiments of the present invention have been describedabove with reference to the accompanying drawings, whilst the presentinvention is not limited to the above examples, of course. A personskilled in the art may find various alternations and modificationswithin the scope of the appended claims, and it should be understoodthat they will naturally come under the technical scope of the presentinvention.

REFERENCE SIGNS LIST

-   1 Radio communication system-   100, 300, 500 User equipment-   110 Radio communication unit (User equipment)-   160, 360, 560 Control unit (User equipment)-   200, 400, 600 Base station-   210 Radio communication unit (Base station)-   280, 480, 680 Control unit (Base station)

The invention claimed is:
 1. A method for controlling a handover from a first base station to a second base station by a user equipment that performs radio communication over a communication channel formed by carrier aggregation of a plurality of component carriers, the method comprising: transmitting, from the first base station to the user equipment, scheduling information of each component carrier of the plurality of component carriers respectively according to a scheduling information transmission method that is a cross scheduling method; commanding, by the first base station, the user equipment to perform the handover of the communication channel formed by carrier aggregation of the plurality of component carriers; causing transfer of a first component carrier of the plurality of component carriers in the first base station to a first transferred component carrier in the second base station, the first component carrier being operated according to the cross scheduling method; and stopping, by the first base station, transmission of the scheduling information of the first component carrier of the plurality of component carriers in the first base station.
 2. The method according to claim 1, further comprising: receiving, by the first base station, measurement information from the user equipment; and determining, by the first base station, whether the handover is necessary according to the measurement information transmitted from the user equipment.
 3. The method according to claim 1, further comprising: causing, after causing the transfer of the first component carrier, transfer of a second component carrier in the first base station to a second transferred component carrier in the second base station, the second component carrier being operated according to the straight scheduling method.
 4. The method according to claim 3, further comprising: causing, before causing the transfer of the second component carrier, transfer of a third component carrier of the plurality of component carriers in the first base station to a third transferred component carrier in the second base station, the third component carrier being operated according to the cross scheduling method, and stopping, by the first base station, transmission of the scheduling information of the third component carrier of the plurality of component carriers in the first base station.
 5. The method according to claim 1, thither comprising transmitting, from the second base station to the user equipment, the scheduling information of the first transferred component carrier in the second base station according to the straight scheduling method after the first component carrier is transferred to the first transferred component carrier in the second base station.
 6. A user equipment, comprising: a radio communication circuit that performs radio communication with a first base station over a communication channel formed by carrier aggregation of a plurality of component carriers; a control circuit that controls a handover from the first base station to a second base station and a quality measuring circuit that measures a channel quality of the communication channel between the user equipment and the first base station, wherein the radio communication circuit is configured to receive, from the first base station, scheduling information of each component carrier of the plurality of component carriers respectively according to a scheduling information transmission method that is a cross scheduling method, and receive a command, from the first base station, to perform the handover of the communication channel formed by carrier aggregation of the plurality of component carriers, and the control circuit is configured to cause transfer of a first component carrier of the plurality of component carriers in the first base station to a first transferred component carrier in the second base station, the first component carrier being operated according to the cross scheduling method, and stop reception of the scheduling information of the first component carrier of the plurality of component carriers in the first base station.
 7. The user equipment according to claim 6, wherein the radio communication circuit transmits measurement information of the channel quality measured by the quality measuring circuit to the first base station.
 8. The user equipment according to claim 6, wherein the control circuit is further configured to cause, after causing the transfer of the first component carrier, transfer of a second component carrier in the first base station to a second transferred component carrier in the second base station, the second component carrier being operated according to the straight scheduling method.
 9. The user equipment according to claim 6, wherein the radio communication circuit is further configured to receive the scheduling information of the first transferred component carrier in the second base station according to the straight scheduling method after the first component carrier is transferred to the first transferred component carrier in the second base station.
 10. A base station, comprising: a radio communication circuit that performs radio communication with a user equipment over a communication channel formed by carrier aggregation of a plurality of component carriers; and a control circuit that controls a handover to another base station by the user equipment, wherein the radio communication circuit is configured to transmit scheduling information of each component carrier of the plurality of component carriers respectively according to a cross scheduling method, the control circuit is configured to command the user equipment to perform the handover of the communication channel formed by carrier aggregation of the plurality of component carriers, such that transfer of a first component carrier of the plurality of component carriers in the base station to a first transferred component carrier in the other base station is caused, the first component carrier being operated according to the cross scheduling method, and the control circuit is further configured to stop transmission of the scheduling information of the first component carrier of the plurality of component carriers in the base station.
 11. The base station according to claim 10, wherein, after the transfer of the first component carrier is caused, transfer of a second component carrier in the base station to a second transferred component carrier in the other base station is caused, the second component carrier being operated according to the straight scheduling method.
 12. A method for controlling a handover from a first base station to a second base station by a user equipment that performs radio communication over a communication channel formed by carrier aggregation of a plurality of component carriers, the method comprising: receiving, by the user equipment from the first base station, scheduling information of each component carrier of the plurality of component carriers respectively according to a scheduling information transmission method that is a cross scheduling method; measuring, by the user equipment, a channel quality of the communication channel formed by carrier aggregation of the plurality of component carriers; transmitting, by the user equipment to the first base station, measurement information of the channel quality; receiving, by the user equipment from the first base station, a command to perform the handover of the communication channel formed by carrier aggregation of the plurality of component carriers; causing transfer of a first component carrier of the plurality of component carriers in the first base station to a first transferred component carrier in the second base station, the first component carrier being operated according to the cross scheduling method; and stopping, by the user equipment, reception of the scheduling information of the first component carrier of the plurality of component carriers in the first base station.
 13. The method according to claim 12, further comprising: causing, after causing the transfer of the first component carrier, transfer of a second component carrier in the first base station to a second transferred component carrier in the second base station, the second component carrier being operated according to the straight scheduling method. 